U.S. patent number 8,169,152 [Application Number 13/017,273] was granted by the patent office on 2012-05-01 for plasma lamp with field-concentrating antenna.
This patent grant is currently assigned to Luxim Corporation. Invention is credited to Marc DeVincentis, Richard Gilliard.
United States Patent |
8,169,152 |
DeVincentis , et
al. |
May 1, 2012 |
Plasma lamp with field-concentrating antenna
Abstract
An electrodeless plasma lamp is described comprising a lamp body
including a solid dielectric material. The lamp includes a bulb
received at least partially within an opening in the solid
dielectric material and a radio frequency (RF) feed configured to
provide power to the solid dielectric material. A conductive
material is provided adjacent to the bulb to concentrate the power
proximate the bulb. The conductive material may be located below an
upper surface of the solid dielectric material. The conductive
material may modify at least a portion of an electric field
proximate the bulb so that the portion of the electric field is
oriented substantially parallel to an upper surface of the lamp
body.
Inventors: |
DeVincentis; Marc (Palo Alto,
CA), Gilliard; Richard (Morgan Hill, CA) |
Assignee: |
Luxim Corporation (Sunnyvale,
CA)
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Family
ID: |
38229002 |
Appl.
No.: |
13/017,273 |
Filed: |
January 31, 2011 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20110181184 A1 |
Jul 28, 2011 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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12755650 |
Feb 1, 2011 |
7880402 |
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11619989 |
May 18, 2010 |
7719195 |
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60756087 |
Jan 4, 2006 |
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Current U.S.
Class: |
315/248; 313/634;
315/39 |
Current CPC
Class: |
H01J
65/044 (20130101); H01J 65/042 (20130101); Y02B
20/00 (20130101); Y02B 20/22 (20130101) |
Current International
Class: |
H05B
41/16 (20060101) |
Field of
Search: |
;315/39,248
;313/231,634,636 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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8148127 |
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Jun 1996 |
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JP |
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1020050018587 |
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Feb 2005 |
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KR |
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WO-2006070190 |
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Jul 2006 |
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WO |
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WO-2006129102 |
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Dec 2006 |
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WO |
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WO-2007079496 |
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Jul 2007 |
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WO |
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WO-2007079496 |
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Jul 2007 |
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WO |
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WO-2007138276 |
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Dec 2007 |
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WO |
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|
Primary Examiner: Le; Tung X
Attorney, Agent or Firm: Schwegman, Lundberg & Woessner,
P.A.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of and claims the benefit of
priority under 35 U.S.C. .sctn.120 to U.S. patent application Ser.
No. 12/755,650, filed on Apr. 7, 2010, and issued as U.S. Pat. No.
7,880,402 on Feb. 1, 2011, which is a continuation of and claims
the benefit of priority under 35 U.S.C. .sctn.120 to U.S. patent
application Ser. No. 11/619,989, filed on Jan. 4, 2007, and issued
as U.S. Pat. No. 7,719,195 on May 18, 2010, which claims benefit of
priority under 35 U.S.C. .sctn.119(e) to U.S. Provisional
Application No. 60/756,087, filed on Jan. 4, 2006, which
applications are incorporated by reference herein in their
entirety.
Claims
What is claimed is:
1. An electrodeless plasma lamp comprising: a metal enclosure
having conductive boundary forming a resonant cavity; a radio
frequency (RF) feed to couple power from an RF power source into
the resonant cavity; a bulb adjacent to an upper surface of the
metal enclosure, the bulb containing a fill that forms a plasma
when power is coupled to the fill, the bulb supported at least
partially within the metal enclosure; and a conductive component
having a lower end and an upper end, the lower end being
electrically coupled to the metal enclosure, and the upper end
being proximate the bulb, wherein the conductive component modifies
at least a portion of an electric field of the resonant cavity
proximate the bulb so that the portion of the electric field is
oriented substantially parallel to the upper surface of the metal
enclosure.
2. The electrodeless plasma lamp of claim 1, comprising a first
conductive component and a second conductive component, the first
conductive component being proximate a first end of the bulb and
the second conductive component being proximate a second end of the
bulb.
3. The electrodeless plasma lamp of claim 2, wherein the upper end
of each of the first and second conductive components is part of an
antenna element to concentrate the electrical field proximate the
bulb.
4. The electrodeless plasma lamp of claim 1, wherein the upper end
of the conductive component is part of an antenna element to
concentrate the electrical field proximate the bulb.
5. The electrodeless plasma lamp of claim 4, wherein the antenna
element is elongate and extends substantially parallel with a
longitudinal axis of the bulb.
6. The electrodeless plasma lamp of claim 5, wherein the antenna
element has a length of between 2-10 mm and a width of between 1-5
mm.
7. The electrodeless plasma lamp of claim 4, wherein the antenna
element is a metal coating.
8. The electrodeless plasma lamp of claim 4, wherein the bulb is
comprises quartz and the antenna element is in contact with the
quartz.
9. The electrodeless plasma lamp of claim 4, wherein the antenna
element intercepts RF fields and creates an alternating current
(AC) that radiates into the bulb.
10. The electrodeless plasma lamp of claim 1, wherein the bulb is
an elongate bulb, the plasma lamp comprising a support having a
longitudinal axis aligned with the longitudinal axis of the
bulb.
11. The electrodeless plasma lamp of claim 1, wherein the lower end
of the conductive component is electrically grounded to the metal
enclosure.
12. The electrodeless plasma lamp of claim 1, wherein the
conductive boundary is an electrically conductive coating on a lamp
body.
13. The electrodeless plasma lamp of claim 1, wherein the
conductive component is spaced apart from the bulb and the RF
feed.
14. The electrodeless plasma lamp of claim 1, wherein a distance
from an end of the bulb to the conductive component is less than 5
mm.
15. The electrodeless plasma lamp of claim 1, wherein a resonant
mode for the resonant cavity is a fundamental resonant mode.
16. The electrodeless plasma lamp of claim 1, wherein the elongate
RF feed is a drive probe.
17. The electrodeless plasma lamp of claim 16, wherein the drive
probe is cylindrical with a diameter greater than 1.5 mm.
18. The electrodeless plasma lamp of claim 16, wherein the drive
probe has a length greater than 10 mm.
19. The electrodeless plasma lamp of claim 1, wherein RF power is
provided to the lamp body at a frequency less than 1 GHz.
20. The electrodeless plasma lamp of claim 1, wherein the
conductive component is cylindrical in cross-section.
21. The electrodeless plasma lamp of claim 1, wherein the
conductive component extends along at least an end portion of the
bulb.
Description
FIELD
The field of the present invention relates to devices and methods
for generating light, and more particularly to electrodeless plasma
lamps.
BACKGROUND
Electrodeless lamps may be used to provide point-like, bright,
white light sources. Because electrodes are not used, they may have
longer useful lifetimes than other lamps. Some plasma lamps direct
microwave energy into an air cavity, with the air cavity enclosing
a bulb containing a mixture of substances that can ignite, form a
plasma, and emit light. However, for many applications, light
sources that are brighter, smaller, less expensive, more reliable,
and have longer lifetimes are desired.
Plasma lamps have been proposed that use a dielectric waveguide
body to reduce the size of the lamp. An amplifier circuit may be
used to provide power to the waveguide body to excite a plasma in a
bulb positioned within a lamp chamber in the waveguide body.
What is desired are lamps with improved brightness and efficiency
which can serve as a light source in products such as large-screen
television sets and digital light processing projection systems.
What is also desired are improved methods for production of plasma
lamps, including manufacture of key components and overall lamp
assembly.
INCORPORATION BY REFERENCE
All publications and patent applications mentioned in this
specification are herein incorporated by reference to the same
extent as if each individual publication or patent was specifically
and individually indicated to be incorporated by reference.
BRIEF DESCRIPTION OF THE DRAWINGS
The novel features of the invention are set forth with
particularity in the appended claims. A better understanding of the
present invention will be obtained by reference to the following
detailed description that sets forth illustrative embodiments, in
which the principles of the invention are utilized, and the
accompanying drawings of which:
FIG. 1 shows a partially exploded perspective view of a plasma lamp
according to an example embodiment including a cylindrical
dielectric waveguide body, a dielectric sleeve insert forming a
lamp chamber with a light-reflecting paraboloidal surface, a bulb
assembly with a tipless bulb, and a field-concentrating
antenna.
FIG. 2 is an exploded perspective view of the FIG. 1 waveguide body
and sleeve.
FIG. 3 is a sectional view of the FIG. 2 waveguide body and sleeve,
taken along line 3-3.
FIG. 4 is a sectional view of a one-piece cylindrical dielectric
waveguide body having a lamp chamber with a light-reflecting
paraboloidal surface.
FIG. 5 is a sectional view of the FIG. 1 bulb assembly according to
an example embodiment.
FIG. 6 is a sectional view of the FIG. 1 bulb assembly bonded to a
metallic holder.
FIG. 7 is a detail perspective view of the FIG. 1
field-concentrating antenna and bulb assembly, and FIG. 6 metallic
holder.
FIG. 8 graphically shows the spatial distribution and intensity of
the electric field in the FIG. 1 waveguide body and lamp chamber in
the absence of the field-concentrating antenna, for a resonant
frequency of 878 MHz.
FIG. 9 graphically shows the spatial distribution and intensity of
the electric field in the FIG. 1 waveguide body and lamp chamber
with the field-concentrating antenna, for a resonant frequency of
878 MHz.
FIG. 10 is a perspective view of a bulb embodiment wherein two
field-concentrating antenna elements are integrated within a quartz
bulb assembly.
FIG. 11 schematically shows an electric circuit for a plasma lamp
having a FIG. 10 bulb assembly.
DETAILED DESCRIPTION
While the present invention is open to various modifications and
alternative constructions, the embodiments shown in the drawings
will be described herein in detail. It is to be understood,
however, there is no intention to limit the invention to the
particular forms disclosed. On the contrary, it is intended that
the invention cover all modifications, equivalences and alternative
constructions falling within the spirit and scope of the invention
as described herein and as expressed in the appended claims.
As shown in FIG. 11, an example embodiment provides a plasma lamp
with a waveguide body 132 comprising a dielectric material. In the
example embodiment of FIG. 11, the waveguide body is configured to
resonate when power is provided to the waveguide body at a
particular frequency. By operating at or near a resonant frequency,
power builds in the waveguide body and can be used to ignite and
sustain a plasma discharge in a bulb 146 adjacent to the waveguide
body 132. Electromagnetically, the waveguide body acts as a
resonant cavity in this example. In example embodiments, the
waveguide body may be configured to resonate in a fundamental mode,
a second order mode, third order mode or other resonant mode.
The size of the waveguide body required to achieve resonance at a
desired frequency generally scales inversely with the square root
of the dielectric constant of the material used. As a result,
materials with a higher dielectric constant may be used to reduce
the size of the lamp. In an example embodiment, the waveguide body
132 has a dielectric constant greater than 1, which is the
dielectric constant of air. For example, alumina, a ceramic having
a dielectric constant of about 9, may be used. In some embodiments,
the dielectric material may have a dielectric constant in the range
of from 2 to 10 or any range subsumed therein, or a dielectric
constant in the range from 2 to 20 or any range subsumed therein,
or a dielectric constant in the range from 2 to 100 or any range
subsumed therein, or an even higher dielectric constant. In some
embodiments, the waveguide body may include more than one such
dielectric material resulting in an effective dielectric constant
for the body within any of the ranges described above.
In this example embodiment, the waveguide body forms a lamp chamber
140 with a tapered wall that reflects light out of the lamp
chamber. In some embodiments, the wall may taper to a vertex and
form a shape having a focal point in the lamp chamber. For example,
the lamp chamber 140 may have a paraboloidal or ellipsoidal shape.
The walls of the lamp chamber may comprise a reflective dielectric
materials (such as alumina) and/or be coated with a reflective
coating such as a thin film, multi-layer dielectric coating. In
this example, the reflective surface is not made from a conductive
material that would prevent or substantially attenuate the
transmission of power from the waveguide body 132 into the lamp
chamber 140. In one example embodiment, a thin film, multi-layer
dielectric coating of multiple layers of silicon dioxide (SiO2) may
be used. Another example embodiment uses layers of titanium dioxide
(TiO2). Typically, coatings used in the present invention have
approximately 10 to 100 layers with each layer having a thickness
in a range between 0.1 micron and 10 microns or any range subsumed
therein.
In example embodiments, a bulb 146 is positioned completely or
partially in the lamp chamber. The bulb contains a fill that forms
a plasma and emits light when power is provided from the waveguide
body to the bulb. The light is reflected from the walls of the lamp
chamber 140 out the front of the lamp. The position of the bulb and
the shape of the lamp chamber may be selected to provide a desired
ray divergence out the front of the lamp. In example embodiments,
the bulb may be positioned above the vertex of the lamp chamber
such that the arc formed in the bulb is near the focal point of the
lamp chamber. In example embodiments, the rays exiting the lamp
chamber may be convergent, parallel or have some other ray
divergence that matches an optical system used with the lamp. For
instance, an optical system with lenses, reflectors and/or light
pipes may be used in a projection display system to provide light
from the light source to a spatial light modulator (SLM). In an
example embodiment, the position of the bulb and the shape of the
lamp chamber may match the ray divergence required by the optical
system used with the lamp (e.g., the optical system for a
projection display). The surface shape may be optimized for the
desired ray divergence using commercial ray-tracing software,
taking into account the finite emission volume of the plasma in the
bulb, geometric constraints imposed by the bulb support structure,
and constraints imposed by manufacturing processes. Suitable
software products include ZEMAX.TM., available from Zemax
Development Corporation of San Diego, Calif., and CODE-V.TM.,
available from Optical Research Associates of Pasadena, Calif. In
example embodiments, the lamp chamber may provide a reflective
surface that approximates a paraboloidal or ellipsoidal shape and
the center of the bulb may be positioned at or near a focal point
for the particular shape. In example embodiments, the arc length of
the plasma is relatively small and the arc may be centered at or
near the focal point of the reflective lamp chamber.
In the example shown in FIG. 11, the bulb 146 may be positioned
completely inside the lamp chamber 140 and may be at or near a
focal point of the lamp chamber. In some alternative embodiments, a
portion of the bulb could be in contact with the walls of the
waveguide body or another material to act as a heatsink. Also, in
alternative embodiments, the end of the bulb could extend outside
of the lamp chamber to isolate it from the region of highest plasma
intensity. However, the example embodiment in FIG. 11 uses a bulb
positioned completely in the lamp chamber and spaced from the
reflective walls of the waveguide in order to provide high
brightness and desired reflection/ray divergence. In example
embodiments, the power is provided along the entire length of the
bulb, so the arc length of the bulb is determined primarily by the
size of the bulb. The bulb has a relatively short inner length and
small volume to provide a small arc length and high power density.
In example embodiments, the arc length may be in the range of from
2 mm to 10 mm or any range subsumed therein. The bulb also has
relatively thick walls to withstand exposure to the plasma
environment. In an example embodiment, the top and bottom of the
bulb have a hemisphere shape with a central cylindrical section
connecting the two ends. In a particular example, the bulb may have
an inner width of about 3 mm, an inner length of about 5 mm, a
volume of about 23.55 mm.sup.3 and a wall width of about 3.5
mm.
Other bulb configurations may be used in other embodiments. For
example, the bulb may be cylindrical with a planar top and bottom
or hemisphere top and bottom; or the bulb may be spherical; or the
bulb may have an oval cross section with curved top and bottom
surfaces. These shapes are examples only and other shapes may be
used as well such as parabolically contoured bulbs or irregularly
shaped bulbs (e.g., hourglass shaped bulbs).
Each of the above bulbs has a length L between the inside walls of
the bulb and an outer length OL. Each of the bulbs also has a width
W between the inside walls of the bulb and an outer width OW. In
bulbs with circular cross sections, the width W is equal to the
inner diameter of the bulb and the outer width OW is equal to the
outer diameter of the bulb. In a spherical bulb, the length and
width are both equal to the diameter. For irregular shaped bulbs,
the inner width may be determined by using the largest interior
width in the region where power is predominantly coupled into the
bulb and the inner length may be determined using the greatest
length between distal ends of the bulb.
In example embodiments, the bulb may be in any of the above shapes
or other shapes and have, for example, an outer width OW in a range
between 2 and 35 mm or any range subsumed therein, an inner width W
in a range between 1 and 25 mm or any range subsumed therein, a
wall thickness in a range between 0.5 and 5 mm or any range
subsumed therein, an inner length L between 3 and 20 mm or any
range subsumed therein. In example embodiments, the bulb volume may
be between 10.47 mm.sup.3 and 750 mm.sup.3 or any range subsumed
therein. The above dimensions are examples only and bulbs with
other dimensions may also be used in embodiments of the present
invention.
Example bulbs in any of the above configurations may comprise an
envelope of transmissive material such as quartz, sapphire or other
solid dielectric. In some embodiments, the bulb envelope may be
formed from a monolithic material. In other embodiments, bulbs may
also be formed by a combination of materials forming an envelope.
For example, a reflective body of ceramic may have an opening
covered by a transmissive window of quartz, sapphire or other
transmissive material. Some bulbs may also be formed in part by
surfaces of the waveguide body and/or other surfaces of a lamp
body. For example, a lamp chamber may be formed in the waveguide
body and covered by a transmissive window of quartz, sapphire or
transmissive material.
Some bulbs may be filled through a small opening that is then
sealed. This can form a surface irregularity, called a "tip" or
"tubulation", where the bulb is sealed. In particular, some bulbs
may be filled through a side wall of the bulb and an irregular side
tubulation may be formed. With high temperatures and high pressures
in an electrodeless bulb, a side tubulation that is too thin may be
susceptible to failure and a side tubulation that is too thick may
introduce optical distortions. Non-uniformities may also cause
localized hot spots that can cause failure. For example, the
pressure inside an electrodeless plasma bulb may be in the range of
from 50 atmospheres to 250 atmospheres or more, or any range
subsumed therein, and the temperature may be greater than 800
degrees Celsius.
In example embodiments, the bulb may be fabricated from a tube of
dielectric material such as quartz using a tipless method that does
not form side tubulation irregularities. The tube is sealed at one
end and a fill is provided through the other end of the tube. The
other end of the tube is then closed with a torch at a point about
one inch longer than the desired inner length. A bulb of this type
can be fabricated with a relatively thick, uniform wall to
withstand the plasma environment and a relatively small interior
volume to confine the plasma arc. Since the surface irregularities
of a tip are avoided, more uniform heat dissipation and more
uniform optical surface for light collection may be achieved. In
example embodiments, this process may be used to form a relatively
thick bulb wall that has a substantially uniform thickness. For
example, the wall thickness may be in the range of about 2.5 mm to
5 mm or any range subsumed therein and the uniformity of the wall
thickness may be within .+-.5-20% of the wall thickness or any
range subsumed therein. In some example embodiments, the uniformity
of the wall thickness may be within .+-.0.25 mm.
The bulb may be positioned in the lamp chamber using a pedestal or
other support. In one example embodiment, a tipless bulb is used
and tubing below the bulb is retained to act as a support for the
bulb. The support may be attached to the wall of the lamp chamber
or may pass through a hole in the waveguide body and be attached to
a separate support. The bulb may be positioned in the lamp chamber
spaced apart from the wall of the lamp chamber and below the
opening at the top of the lamp chamber. As described above, the
bulb may be at or near a focal point for the lamp chamber. In some
embodiments, the distance from the interior of the bulb to the
bottom of the lamp chamber may range from about half the inner
length of the bulb to twice the inner length of the bulb or more.
In some example embodiments, this distance may range from 2 mm to
25 mm or more, or any range subsumed therein. In some embodiments,
the closest distance from the interior of the bulb to the walls of
the lamp chamber may also be from 2 mm to 25 mm or more, or any
range subsumed therein. In some embodiments, the distance from the
interior of the bulb to the top of the lamp chamber may range from
about half the inner length of the bulb to three times the inner
length of the bulb or more. In some example embodiments, this
distance may range from 2 mm to 40 mm or more, or any range
subsumed therein. In some embodiments, the focal point and position
of the bulb arc is closer to the bottom vertex of the lamp chamber
than to the top opening. The above dimensions are examples only and
bulb configurations with other dimensions may also be used in
embodiments of the present invention.
A power source, such as amplifier 138, may be coupled to the
waveguide body to provide power at a frequency in the range of 50
MHz to 30 GHz or any range subsumed therein. The amplifier 138 may
be coupled to a drive probe 134 to provide power to the waveguide
body. The drive probe may be inserted into an opening formed in the
waveguide body and may be in direct contact with the waveguide body
to effectively couple power into the waveguide body. A feedback
probe 136 may be coupled to the waveguide body and the amplifier to
obtain feedback from the waveguide body and provide it to the
amplifier. The feedback probe may be inserted into an opening
formed in the waveguide body and may be in direct contact with the
waveguide body to effectively obtain feedback from the waveguide
body.
The outer surfaces of the waveguide body 132 may be coated with a
conductive material. In example embodiments, the coating may be
metallic electroplating. In other embodiments, the coating may be
silver paint or other metallic paint. The paint may be brushed or
sprayed onto the waveguide body and may be fired or cured at high
temperature. In an example embodiment, the holes where the probes
are inserted are not coated with the conductive coating in order to
allow power to be effectively coupled into the waveguide body and
similarly the walls of the lamp chamber are not coated to allow
power to be coupled from the waveguide body into the lamp chamber
140. Since the lamp chamber 140 may be substantially larger than
the bulb 146 in order to provide the desired reflective properties,
some embodiments may use conductive material adjacent to the bulb
to concentrate radio frequency power near the bulb. As shown at 142
and 144 in FIG. 11, a conductive material may be external to the
interior of the bulb, but may extend very close to the interior of
the bulb. For example, the material may be about 1 mm from the
interior of the bulb. In other example embodiments, this distance
may range from 0.1 mm to 5 mm or any range subsumed therein. In
some embodiments, this distance may be less than the thickness of
the bulb wall and the conductive material may extend into the bulb
wall. The conductive material acts as an antenna that concentrates
power near the bulb. To avoid arcing and oxidation, the conductive
material may be enclosed in a dielectric material such as quartz.
In this example, the conductive material may be hermetically sealed
in a dielectric material, at least in the region adjacent to the
bulb where the power densities are the highest. In one example
embodiment, the conductive material is a thin foil of molybdenum or
other conductive material. For example, the foil may have a
thickness of about 100 microns or less. In some embodiments, the
foil may have a thickness of about 20 microns or less. A very thin
conductive material such as molybdenum foil may be advantageous,
because it can be sealed in a dielectric material such as quartz or
even penetrate the bulb wall without causing damage due to thermal
expansion of the conductive material inside the dielectric
material. The conductive coating on the outer surface of the
waveguide body 132 and the conductive antennas 142 and 144 are
grounded. In example embodiments, a common ground may be provided
for these elements.
In the example embodiment of FIG. 11, the waveguide body is
configured to resonate when power is provided by the amplifier 138
to the drive probe at a particular frequency. However, the ignition
of the plasma in the bulb and heating of the bulb and the waveguide
body may cause resonant conditions to change (for example, due to
changes in the load characteristics and thermal expansion of the
bulb and waveguide body). The feedback adjusts to changing lamp
conditions to sustain oscillation. A phase-shifter PS1 may be used
to adjust the phase of the signal as lamp conditions change to
reduce reflection of power from the waveguide body 132 and maintain
efficient coupling of power. The phase may also be adjusted during
the ignition process to over couple power to the waveguide body for
a short period of time in order to spike the power to expedite
initial ignition of the plasma in the bulb. The phase-shifter may
be controlled by a microcontroller MC1 or other control circuit
during the startup process and steady state operation to achieve
desired operating characteristics.
Additional details regarding example embodiments will now be
described with reference to FIGS. 1-10. Referring to FIGS. 1, 2 and
3, a plasma lamp 20 according to an example embodiment includes a
cylindrically-shaped dielectric waveguide body 22 having a
generally circular outer surface 22S coated with electrically
conductive material 23, a bore 24 determined by a generally
circular surface 24S, and opposed generally parallel upper and
lower surfaces 22U, 22L coated with electrically conductive
material 23. A cylindrical dielectric sleeve 30 having an outer
surface 30S and a paraboloidal lamp chamber 32 determined by a
surface 32S is closely received within bore 24. The dielectric
sleeve 30 forms the desired reflective lamp chamber 32, but is
fabricated as a separate insert that can fit into the bore 24 in
waveguide body 22 for ease of manufacture. While FIGS. 1, 2 and 3
illustrate how the dielectric sleeve 30 would be inserted into the
bore 24, the top of the dielectric sleeve 30 would be aligned with
the top of the waveguide body 22 when fully assembled in this
example embodiment. Waveguide body 22 and sleeve 30 can be made of
any low-loss, high dielectric constant material, although alumina
is used in a particular example embodiment.
Plasma lamp 20 further includes a metallic top adapter plate 34
having a lower surface 34L to which is electrically grounded an
electromagnetic field-concentrating antenna 40, and a metallic
bottom adapter plate 36 having upper and lower surfaces 36U, 36L. A
generally cylindrical, metallic holder 38 having a lip 38L is
attached to surface 36U. A bulb assembly 50 having upper and lower
ends 50U, 50L is positioned symmetrically along the common
longitudinal axis of waveguide body 22 and lamp chamber 32. As
described above, the bulb assembly may be formed from a tube of
dielectric material sealed near one end to form a bulb 56 within
the bulb assembly. The other end may comprise a length of tube that
supports the bulb assembly. The tube is inserted through holes 25H
and 31H to position the bulb in the lamp chamber 32. Antenna 40 is
proximate to but does not touch bulb assembly upper end 50U. Bulb
assembly end 50L is closely received within and bonded to holder
38. Surface 36L is attached to a housing 60 including a circuit
board 62 including a microwave amplifier 63 and associated
circuitry, and a heatsink-radiator 64. Adapter plates 34 and 36 are
bolted to housing 60. Coaxial feeds leading to drive probe 66 and
feedback probe 68 extend through circuit board 62 and are received
within holes 66H, 68H, respectively, in surface 22L of waveguide
body 22. The adapter plate holds the bulb assembly 50 in position
relative to the through holes for the probes and acts as a
convenient mechanism for aligning the bulb assembly and probes with
the waveguide body 22 and dielectric sleeve 30. Thus, plasma lamp
20 is a single unit integrating the waveguide body 22, lamp chamber
32, antenna 40, bulb assembly 50, circuit board 62, and
heatsink-radiator 64.
Still referring to FIG. 3, waveguide body 22 has a lower solid
portion 25 with a central hole 25H, and sleeve 30 has a lower solid
portion 31 with a central hole 31H aligned with hole 25H. Bulb
assembly 50 is closely received through holes 25H and 31H. The
outer surface 30S of sleeve 30 is coated with an alumina adhesive
30A which bonds to the bore surface 24S. Chamber surface 32S is
coated with a plurality of multi-layer dielectric coatings 32C
designed to reflect the visible spectrum, as described above.
Although chamber 32 is shown to have the shape of a parabola
rotated about the longitudinal axis, its shape can be any one of a
number of similarly rotated conic curves, a plurality of
discretely-faceted surfaces, or surfaces of arbitrary shape as
optimized by optical ray-tracing analysis.
Alternatively, a one-piece waveguide body and lamp chamber can be
used. FIG. 4 shows a cylindrical, dielectric waveguide body 70
bounded by an outer surface 70S and lower and upper surfaces 70L,
70U. Body 70 includes a lamp chamber 72 determined by a
paraboloidal surface 72S coated with a plurality of dielectric
coatings 72C. The bottom of chamber 72 is in communication with a
hole 74 sized to closely receive a bulb assembly. Holes 76H, 78H
are sized to closely receive, respectively, a drive probe and
feedback probe.
A plasma bulb must operate at elevated wall temperatures
(>800.degree. C.) and internal pressures (between 50 and 250
atmospheres). Fabrication methods which use a separate fill-tube
("tip") to introduce light-emitting material into a bulb made from
tubing stock, may result in a very thin closure where the fill-tube
was attached to the tubing, typically 1-1.5 mm in thickness, or
other surface irregularities. A bulb with such a "thin spot" may
have reliability problems at such high temperatures and pressures
and could potentially rupture. Also, using a tip may leave an
optical blemish on the bulb surface which decreases light
throughput. Also, thermal asymmetries may develop which can affect
consistent evaporation of halides (and therefore consistent color
and lumens as well as consistent warm-up time to full brightness)
from bulb to bulb.
In some example lamps, the waveguide body may be narrowed in the
region adjacent to the bulb to limit the length of the plasma
region. In the example embodiments shown in FIGS. 1-4, the bulb is
positioned in the lamp chamber spaced apart from the walls to
provide desired reflection and ray divergence. In these example
embodiments, the entire internal volume of the bulb is exposed to
power coupled into the lamp chamber from the waveguide body. As a
result, the length and diameter of the plasma region in the bulb is
limited by the dimensions of the bulb and not the shape of the
waveguide body adjacent to the bulb. In order to provide a short
arc, it is desirable to make the interior bulb volume very short
and narrow in the example embodiments of FIGS. 1-4. In one example
embodiment, the internal length of the bulb may be about 5 mm and
the internal width may be about 3 mm.
In some example lamps, the waveguide body may also provide a
heatsink to avoid excessive bulb temperatures. In the example
embodiments shown in FIGS. 1-4, the bulb is spaced apart from the
walls. Without a heatsink in contact with the bulb, the bulb must
survive considerably higher power loadings per unit surface area
(i.e., considerably higher bulb temperatures). In order to achieve
this in the example embodiments of FIGS. 1-4, the wall of the bulb
is made relatively thick with as uniform a wall thickness as
practical. A thick-wall bulb will distribute heat far more
uniformly than will a thin-wall bulb, or one with wall thickness
irregularities. These properties may be achieved by forming a
tipless bulb from a tube of dielectric material, such as quartz, in
example embodiments. An additional benefit to fabricating a tipless
bulb from a tube is a convenient mounting stem formed from the same
piece of tubing stock, which can be easily tailored to the desired
length to support the bulb in the lamp chamber.
An example bulb 56, shown in FIG. 5, is fabricated from tubing
stock, such as quartz, without the need for a separate tip. An
example method of fabricating a plasma bulb such as bulb 56 is as
follows: A tube, typically cut from a piece of longer stock, is
selected having an outer diameter and wall thickness about the same
as those of the desired bulb, and a length that depends on the
desired length of the bulb assembly. In one example, the outer
diameter is about 7 mm and the wall thickness is about 2 mm. The
tube is cleansed and rinsed. Using a hydrogen-oxygen torch, one end
of the tube is rounded and smoothed on a glass-lathe to form an
approximately hemispherical closure (52, FIG. 5) with an inner
radius about the same as the tube inner radius, and a wall
thickness about the same as the tube wall thickness. In other
embodiments, other heat sources or mechanisms may be used to close
the end of the tube. The partially formed body is re-cleansed. The
open end (54, FIG. 5) of the tube is filled with a "light-emitter".
Typically, mercury or indium bromide is used. Other light-emitter
substances may be used in other embodiments, including iodides,
bromides and/or chlorides of lithium, sodium, potassium, rubidium,
cesium, strontium, scandium, cerium, praseodymium, neodymium,
gadolinium, dysprosium, holmium, hafnium, thallium, lutetium,
yttrium, erbium, thulium, terbium and europium. Pure metals and
halogens can also be introduced into the tube to achieve desired
properties. The filled tube is attached on a high vacuum system
with a standard fitting of the appropriate diameter (e.g., an
UltraTorr.TM. fitting available commercially from SwageLok Inc.)
and evacuated. The tube is then backfilled with the desired
"starting gas" at the desired pressure. Typically, the starting gas
is argon, although neon, krypton or xenon may be used in other
embodiments. In example embodiments, the gas pressure is in a range
between about 10 and 500 Torr or any range subsumed therein. The
tube is then closed with a hydrogen-oxygen torch at a point (55,
FIG. 5) approximately one inch longer than the desired inner length
(about 5 mm) of the bulb, to form a bulb (56, FIG. 5). The sealed
bulb may be frozen in the tube using a cryogenic material such as a
liquid nitrogen bath, so the stem can be heat collapsed at negative
pressure up to the desired bulb edge without evaporation of the
internal fill materials creating internal positive pressure. The
bulb and attached stem (58, FIG. 5) are removed from the vacuum
system. If necessary, the stem can be trimmed to the correct
length. Residual silica dust is removed from the lamp surfaces
using hydrofluoric acid or buffing compound.
The bulb assembly may be mounted to a structure exterior to the
dielectric waveguide body, such as adapter plate 36 in FIG. 1. The
stem acts as a bulb support and extends through holes 25H and 31H
as shown in FIG. 3 to position the bulb 56 at the desired location
in the lamp chamber 32. This provides an easy way to secure and
align the bulb and position it in the lamp chamber. In alternate
embodiments, a bulb support may be bonded to the bottom of the lamp
chamber or the waveguide body with a sintered alumina-powder layer
or other adhesive material to hold the bulb in the desired
position.
FIG. 6 shows the stem 58 mounted within holder 38 and bonded to the
holder with high temperature, epoxy cement. Holder 38 includes a
screw 39 for attachment to bottom adapter plate 36. The overall
length of the bulb and stem is dependent on the desired position
for the bulb in the lamp chamber. In example embodiments, this may
depend upon the focal length of the lamp chamber's reflecting
surface and details of the mounting design.
In example embodiments, the dielectric waveguide body 22 has one or
more resonant modes each manifested as a certain spatial-intensity
distribution of radio frequency (RF) field confined within the
body. A suitably designed antenna within the lamp can intercept the
RF field in the lamp chamber to create AC currents therein. The AC
current in the antenna can in turn radiate into a partially
enclosed space in which the bulb is closely received. The
dimensions of this space must be small compared to the RF field
wavelength; the net effect is a concentration of RF field in the
enclosed space, i.e., space proximate to the bulb. A conductive
material adjacent to the bulb may be used to form an antenna of
this type and thereby concentrate the RF field near the bulb.
Referring to FIG. 7, field-concentrating antenna 40 is a "top"
antenna having a wire cage 42 forming a tight semi-enclosed space
around bulb 56 at a separation in a range from about 0.1 mm to 5.0
mm or any range subsumed therein. Cage 42 is attached to a rigid
lead-wire 44 which maintains the position of cage 42 and forms a
common electrical ground with coatings 23, adapter plates 34, 36,
and housing 60. In example embodiments, such antennas may be
fabricated from a conductor with a high melting point having an
electrical conductivity greater than 10.sup.7 Siemens/meter.
Example embodiments may use conductive materials such as nickel,
platinum, molybdenum and titanium. In addition to machining,
antennas can be fabricated using stamped sheet-metal or even wire.
In example embodiments, the antenna thickness is selected so that
it is thick enough to resist melting, and thin enough to avoid
blocking an unacceptable fraction of light. Also, in example
embodiments, the antenna thickness is selected to be significantly
larger than the RF penetration depth ("skin depth"). In example
embodiments, the thicknesses may be in the range of from about 0.1
to 2 mm or any range subsumed therein. In example embodiments, the
antenna is grounded to a conductively-coated surface of the
dielectric waveguide body. Various physical shapes for the antenna
can be used to achieve these properties. A top antenna, bottom
antenna, or both can be used. In example embodiments, the enclosure
can take the form of a wire cage, fork or ring. Other shapes can
also be used in other embodiments. The shapes of a top and bottom
antenna can be different in some embodiments. Design of such
antennas can be optimized using commercial electromagnetic modeling
software such as HFSS.TM., available from Ansoft, Inc. of
Pittsburgh, Pa., and FEMLAB.TM., available from COMSOL, Inc. of
Burlington, Mass. In these example embodiments, the
field-concentrating antenna is not physically connected to the
drive or feedback probe.
FIG. 8 shows a design simulation, using the HFSS.TM. software
package, of the electric field spatial-intensity distribution in a
cylindrical alumina waveguide body 80 resonating at a frequency of
878 MHz. Body 80 includes a paraboloidal lamp chamber 82 into which
is inserted a bulb assembly 84 with a bulb 86, and contains an
inserted drive probe 88. For simplicity, the feedback probe is not
simulated because its coupling to the field (and hence its
perturbation on the field) is small by design. The arrows point in
the direction of the electric field; their lengths indicate
relative intensity. FIG. 9 shows the electric field
spatial-intensity distribution when a cage 92 of a
field-concentrating antenna 90 forms a tight semi-enclosed space
around bulb 86 at a separation of 1 mm. The field-concentrating
effect of antenna 90 is evident. Such simulations predict that the
ratio of field strength in the center of a bulb with and without an
antenna is in a range from about 1.05 to 10.
FIG. 10 shows an example bulb assembly 100 having a
"top-and-bottom" antenna configuration. In this example embodiment,
the antennas are enclosed in a dielectric material, such as quartz
tubing extending from the top and bottom of the bulb. This helps
avoid problems that can lead to failure of a bare antenna. A bare
antenna may have failure mechanisms due to arcing to a conducting
surface, and/or rapid oxidation or melting due to high RF currents
flowing through the antenna(s) and/or high temperature conducted or
radiated from the bulb. Bulb assembly 100, which is fabricated from
quartz tubing, includes a central bulb 102 disposed between upper
and lower stems 104, 106. Opposed "top" and "bottom" antenna
elements 108, 110 are spot-welded, respectively, to lead-wires 112,
114 which exit the bulb through tubing openings 116, 118,
respectively. Antenna elements 108, 110 are connected to a common
electrical ground. At each bulb assembly end, multiple lead-wires
may be used to improve both electrical and thermal conductivity.
Antenna elements 108, 110 are sealed in a dielectric material, such
as quartz, and lead-wires 112, 114 are fixed in position when the
bulb 102 is formed by sealing the tubing at locations 119A, 119B.
Lower stem 106 is closely received within and bonded to metallic
holder 120 which includes a screw 122 for attachment to bottom
adapter plate 36. Antenna elements may be strips of molybdenum
micro-foil typically having a thickness of about 0.001 inch, and a
length in a range of 2 to 10 mm, or any range subsumed therein, and
a width in a range of 1 to 5 mm, or any range subsumed therein. The
use of very thin foils and spot-welding prevents cracking of the
quartz-enclosing seals as well as failure due to expansion mismatch
of any diameter of metal wire capable of conducting the RF currents
to which the antennas are exposed in the high temperature
environment. The simple geometry of a bulb assembly such as
assembly 100 makes it easier to fabricate than a bulb assembly such
as assembly 50. The above antenna configurations and dimensions are
examples only and other configurations may be used in other
embodiments.
A bulb assembly such as bulb assembly 100 can be fabricated either
according to the "one-piece" method described above for bulb
assembly 50, or by presealing the antenna elements in separate
stems which are then fused to the bulb. The following summarizes
example bulb assembly fabrication processes for both one- and
two-stem/antenna configurations: A finished bulb assembly can be
produced with a single stem (for attachment to a dielectric
waveguide body and/or lamp chamber by cement or other means), or
with a stem at each end for enclosing and positioning an antenna at
either end or both ends. As described above, in example
embodiments, the antenna may be a molybdenum micro-foil in close
proximity to the bulb, with outer conducting wire(s) spot-welded to
the micro-foil and attached to an electrical ground. If there is to
be a sealed antenna at the upper end, that antenna may be
hermetically sealed into the upper stem using a "shrink" seal
process. This is done by presealing in a separate stem, later to be
fused to a partial bulb assembly including the bulb with the open
lower stem already attached, but before the fill material(s) are
inserted and before a second antenna, if used, is sealed into the
lower stem. Alternatively, the upper antenna can be sealed into one
arm of a preformed assembly with an open stem on each end using a
shrink seal process. The bulb assembly (with the upper stem
heremetically sealed and attached if an upper antenna is to be
used; otherwise without an upper arm, but with the bulb assembly
sealed everywhere except for an open lower stem) is then filled
under clean conditions through the lower stem with the required
solid and/or liquid fill material(s). If an antenna sub-assembly
(e.g, the antenna element and lead-wire(s)) in the stem is to be
mounted in the dielectric waveguide body or lamp chamber, the
antenna with the attached lead-wire(s) are then inserted into the
lower stem and aligned into axial position. The bulb assembly is
then placed at an appropriate station for filling the lower stem
and bulb with rare gas. The fill material(s) and all parts of the
antenna sub-assembly are completely enclosed in the bulb/lower stem
and the bulb assembly is attached to the gas-filling apparatus by
an O-ring or other suitable means. After (negative gauge pressure)
gas-filling, the lower stem is "long-tipped" (i.e., collapsed by
heating at a distance beyond the inserted length of the
foil/lead-wire(s) sub-assembly). The hermetically sealed structure
is then placed in an apparatus which immerses the bulb assembly
into a cryogenic environment (e.g., liquid nitrogen) so that final
sealing of the lower stem (with or without the antenna
sub-assembly) can be accomplished by heat collapsing the stem at
negative pressure up to the bulb edge without evaporation of the
fill material(s) creating positive pressure (which would prevent
collapsing of the quartz under applied heat and final closure).
Each bulb assembly stem is then cut to an appropriate length,
exposing the lead-wire(s) for attachment to the common electrical
ground. A metal (or, if lead-wires protrude, a slotted metal or
non-conducting end cap) can be attached to the lower stem if needed
for precise alignment of the bulb.
* * * * *